Vehicles operating on roadways are often weighed to determine the axle weight and the total weight of the vehicle. In some operations, the weight of the vehicle is important to ensure compliance with weight restrictions on public roadways. Owners and operators of vehicles exceeding maximum legal weights are subject to fines, and in the event of an accident, can be subject to substantial financial liability for operating a vehicle exceeding the maximum legal weight. It is therefore desirable to weigh trucks and other vehicles which will be operating on public roadways.
One way vehicles are weighed is by driving the vehicle onto a static scale that is large enough to accommodate the entire vehicle. While such a scale is typically accurate to determine the load carried by the vehicle, such scales are very large and very expensive, and must be capable of accommodating and accurately measuring substantial weights. Furthermore, such scales do not enable determination of the weight carried by each individual axle of the vehicle.
Another weighing system involves driving the vehicle onto a smaller scale sized to weigh each individual axle. Such scales typically require driving the axles onto the scale individually, and stopping to weigh each axle. As the vehicle stops and restarts, the load carried by the vehicle can shift, resulting in weight readings that are not accurate. Additionally, the suspension of the vehicle can shift during the stopping and restarting of the vehicle, further reducing the accuracy of the weight measurement.
Some vehicle scales, such as the axle scale 20 shown in FIG. 1, are designed to weigh each axle of the vehicle as the vehicle drives over the scale 20 at a constant speed. Such scales typically include four load cells (only two are shown in FIG. 1) 24, 26 positioned underneath the scale, one located in each corner positioned inwardly from the outer edges of the weighing portion, also known as the active section 28. As the vehicle tires 32 pass over the active section of the scale, the load cells 24, 26 are compressed, and generate a load signal representing the weight of the axle passing over the scale 20.
However, as the vehicle tires 32 first roll onto the active section 28, the downward force 36 from the vehicle is outside an area between the load cells 24, 26 located under the active section 28. A moment 40 is therefore generated, whereby the load cells 26 opposite the tires 32 are urged upwardly 38 while the load cells 24 nearest the tires 32 are urged downwardly 39. A moment is generated in the opposite direction as the wheels pass the load cells under the opposite side of the active section. These moments affect the accuracy of the weight measurement, and make it more difficult to obtain a weight reading of the moving axles.
Additionally, in a typical axle scale 20, the load cells 24, 26 are designed to measure a compression force generated by the additional weight of the vehicle axle on the scale. The load cells 24, 26 support the platform of the active section 28 of the scale 20 from underneath the platform, as shown in FIGS. 1 and 2. As the vehicle tires 32 roll onto the platform, the momentum of the wheels urges the platform in the horizontal direction of movement of the vehicle, illustrated by arrow 44. This movement generates a moment 48 about the support of the load cells 24, 26, resulting in forward and downward movement of the active section 28 relative to the support of the load cells 24, 26. The downward force 36 from of the weight of the load further supplements the forward and downward movement of the active section 28. This forward and downward movement can result in inaccurate weighing of the vehicle.
In some scales, the platform is designed to abut against a stop located outside the active section of the scale in order to arrest this forward and downward movement, and the scale then settles back into the natural position. While such a solution is effective to stop the forward movement, it takes time for the platform to move against the stop and stabilize, increasing the time the axle must be on the scale to produce a weight reading.
Installation of a vehicle scale is a time consuming, cumbersome, and expensive process. Significant construction equipment is required to excavate the scale site, install a frame, and cast concrete pads within the scale site. Furthermore, specialized tools and knowledge are required to install and calibrate the load cells and the moving parts of the scale at the site. If installation is not performed precisely, the scale readings can be subject to substantial errors.
Once installed, typical vehicle scales require routine maintenance to remove objects and debris that can pass through a gap 56 (FIG. 1) between the active section 28 and the surrounding area. Performing this maintenance requires removal of the active section of the scale to clean the area underneath the platform. The active section of typical vehicle scales are difficult to remove, since the load cell, which is located underneath the platform, must be decoupled from either the base or the platform. Additionally, removal and replacement can sometimes require recalibration of the scale, which generally must be performed by a trained specialist.
A scale for heavy loads that has improved measurement accuracy is therefore desirable. Furthermore, it would be desirable to produce a scale for heavy loads that is simpler to install and maintain.